Surface acoustic wave device

ABSTRACT

In a surface acoustic wave device, electrode films constituting at least one IDT are disposed on a piezoelectric substrate, and an SiO 2  film is arranged on the piezoelectric substrate so as to cover the electrode films. The film-thickness of the electrode films is in the range of about 1% to about 3% of the wavelength of an excited surface acoustic wave.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface acoustic wave deviceand, in particular, to a surface acoustic wave device in which aninsulating film is arranged to cover electrode films constituting atleast an IDT (interdigital transducer).

[0003] 2. Description of the Related Art

[0004] In known RF surface acoustic wave filters, LiTaO₃ substrates andLiNbO₃ substrates are used as piezoelectric substrates. To improve thefrequency-temperature coefficients of the piezoelectric substrates,different structures have been proposed in which electrode filmsconstituting IDTs are formed on piezoelectric substrates, andthereafter, SiO₂ films are formed on the piezoelectric substrates so asto cover the electrode films (e.g., Japanese Unexamined PatentApplication Publication Nos. 2-37815 (Patent Document 1), 8-265088(Patent Document 2), and 9-186542 (Patent Document 3), and WO96/4713(Patent Document 4)).

[0005] The surface acoustic wave devices having SiO₂ films formedtherein to improve the frequency-temperature coefficients as describedabove have problems in that the characteristics are deteriorated sincethe upper surfaces of the SiO₂ films have convex portions and concaveportions.

[0006] On the other hand, in the case in which the upper surfaces of theSiO₂ films are flattened as described in Patent Document 4, thereflection on electrode fingers or the like is reduced, so that thecharacteristics are improved to some degree. However, a process offlattening the upper surfaces of SiO₂ films is required, or a step offorming an SiO₂ film of which the upper surface is flat becomesnecessary. Thus, the production method tends to become complicated.

SUMMARY OF THE INVENTION

[0007] In order to overcome the problems described above, preferredembodiments of the present invention provide a surface acoustic wavedevice having an insulating film arranged on a piezoelectric substrateso as to cover an electrode film, which has superior resonancecharacteristics, filter characteristics, and frequency-temperaturecoefficient, and which can be are formed by a simple process.

[0008] According to a preferred embodiment of the present invention, asurface acoustic wave device includes a piezoelectric substrate,electrode films disposed on the piezoelectric substrate and constitutingat least one IDT, and a sputtered insulating film arranged on thepiezoelectric substrate so as to cover the electrode films and theinsulating film having convex portions and concave portions on the uppersurface thereof, the film-thickness of the electrode films being in therange of about 1% to about 3% of the wavelength of an excited surfaceacoustic wave.

[0009] The insertion loss is reduced, and moreover, the filtercharacteristics and the resonance characteristics are superior, sincethe film-thickness of the electrode films is in the range of about 1% toabout 3% of the wavelength of an excited surface acoustic wave. If thefilm-thickness exceeds about 3%, the insertion loss increases. If thefilm-thickness of the electrode films is less than about 1%, theconductor loss in the electrodes becomes large.

[0010] According to a preferred embodiment of the present invention, theinsulating film has convex portions and concave portions on the uppersurface thereof, That is, the convex portions and concave portions existin which the portions of the insulating film positioned above theelectrode films are convex compared to the other portions of theinsulating film.

[0011] Preferably, the insulating film is made of SiO₂. Thereby, thefrequency-temperature coefficient TCF of the surface acoustic wavedevice is effectively improved.

[0012] Preferably, the film-thickness of the insulating film made ofSiO₂ is in the range of about 15% to about 40%, more preferably, atleast about 30% of the wavelength of the surface acoustic wave. Thereby,the frequency-temperature coefficients TCF is reduced by at least about50% compared to that of such a device in which no SiO₂ is formed.

[0013] Also, preferably, the piezoelectric substrate has afrequency-temperature coefficient TCF which is in the range of about−100 ppm/° C. to about −10 ppm/° C. Thus, the combination of thepiezoelectric substrate with the insulating film such as the SiO₂ filmor other suitable film effectively improves the frequency-temperaturecoefficient TCF.

[0014] Preferably, the piezoelectric substrate is a rotation Y-cutX-propagation LiTaO₃ substrate or a rotation Y-cut X-propagation LiNbO₃substrate. When the piezoelectric substrate is used, thefrequency-temperature coefficient TCF is effectively improved while theresonance characteristics and the filter characteristics are scarcelydeteriorated. Preferably, the piezoelectric substrate is a rotationY-cut X-propagation LiTaO₃ substrate or a rotation Y-cut X-propagationLiNbO₃ substrate, which has a cut-angle in the range of about 0° toabout 160°. Thereby, the frequency-temperature coefficient TCF is moreeffectively improved.

[0015] Preferably, the piezoelectric substrate is a rotation Y-cutX-propagation LiNbO₃ substrate, and the surface acoustic wave is a Lovewave which generates no attenuation. Thereby, the surface acoustic wavedevice has a small attenuation and superior characteristics.

[0016] Also, preferably, the surface acoustic wave device is providedwith a reflector defined by the electrode films. Thus, the surfaceacoustic wave device may be a resonator, a resonator type filter orother suitable device which is provided with the reflector, or may be anend surface reflection type surface acoustic wave device which utilizesreflection on the two opposed end surfaces of a piezoelectric substrate.

[0017] In the surface acoustic wave device of various preferredembodiments of the present invention, the electrode films may be made ofdifferent metal materials. Preferably, the electrode films are made ofAl or an Al alloy. Thereby, the reflection coefficient is enhanced, anda superior resonance characteristic is obtained.

[0018] Also, preferably, the electrode films are made of a metal or analloy having a higher density than Al. Thus, even if the electrodefilm-thickness is reduced, a high electromechanical coupling coefficientcan be obtained. Moreover, the reflection coefficient becomes large, andthe number of electrode fingers of IDT can be reduced. Thus, the size ofthe surface acoustic wave resonator can be reduced. In addition, thesurface acoustic wave device utilizing a Love wave can be easilyproduced.

[0019] Preferably, according to a preferred embodiment of the presentinvention, a one-port type surface acoustic wave resonator is provided.Also, preferably, a resonator type filter, a ladder type filter, or alattice type filter may preferably be provided.

[0020] In the case in which the piezoelectric property is enhanced byformation of a piezoelectric film made of Ta₂O₅, ZnO or other suitablematerial, or the surface protecting function is enhanced by formation ofthe insulating film made of another insulating material, thecharacteristics of the surface acoustic wave device are prevented frombeing deteriorated. In addition, according to preferred embodiments ofthe present invention, the surface of the insulating film may haveconvexities and concavities, so that a complicated process is notrequired for the formation of the insulating film.

[0021] Other features, elements, characteristics, steps and advantagesof the present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1A is a schematic plan view of a surface acoustic waveresonator according to a preferred embodiment of the present invention,

[0023]FIG. 1B is a cross-sectional view of the surface acoustic waveresonator, taken along line A-A in FIG. 1A;

[0024]FIG. 2 is a graph showing the change of the resonancecharacteristics of surface acoustic wave resonators in which Alelectrodes having a thickness of about 8% of the wavelength of a surfaceacoustic wave are formed on Y-cut X-propagation LiTaO₃ substrates, andSiO₂ films having different film-thicknesses are formed;

[0025]FIG. 3 is a schematic cross-sectional view of a surface acousticwave resonator which exhibits the measurement results shown in FIG. 2;

[0026]FIGS. 4A and 4B show electron microscopic photographs of thesurface acoustic wave resonators exhibiting the characteristics in FIG.2 which show the states of the surfaces of the insulating films of theresonators as an example;

[0027]FIG. 5 is a graph showing the change of the resonancecharacteristics of surface acoustic wave resonators in which Alelectrodes having a thickness of about 2% of the wavelength of a surfaceacoustic wave are formed on 36° Y-cut X-propagation LiTaO₃ substrates,and SiO₂ films having different film-thicknesses are formed;

[0028]FIGS. 6A and 6B are electron microscopic photographs of surfaceacoustic wave resonators having the characteristics shown in FIG. 5,which show the convex portions and the concave portions on the surfacesof the SiO₂ films;

[0029]FIG. 7 is a graph showing the change of the insertion losses ofladder type filters in which the thickness of an electrode film and thatof an SiO₂ film are varied;

[0030]FIG. 8 is a graph showing the change of the frequency-temperaturecoefficient TCFs obtained when SiO₂ films having differentfilm-thicknesses are formed on LiTaO₃ substrates with differentcut-angles;

[0031]FIG. 9 is a graph showing the change of the frequency-temperaturecoefficient TCFs obtained when SiO₂ films having differentfilm-thicknesses are formed on LiNbO₃ substrates with differentcut-angles;

[0032]FIG. 10 is a graph showing the relationships between thefilm-thicknesses of electrodes of a metal heavier than Al and theelectromechanical coupling coefficients of leaky surface acoustic wavesobtained when the electrodes made of a metal heavier than Al and havingdifferent film-thicknesses are formed on LiNbO₃ substrates having anEuler's angle (0, 154.0, 0);

[0033]FIG. 11 is a graph showing the relationships between thefilm-thicknesses of electrodes and the electromechanical couplingcoefficients of Rayleigh waves and leaky surface acoustic waves obtainedwhen the electrodes made of a metal heavier than Al and having differentfilm-thicknesses are formed on LiNbO₃ substrates having an Euler's angle(0, 131.0, 0);

[0034]FIG. 12 is a graph showing the relationships between the thicknessof IDTs and the reflection coefficients obtained when the IDTs made ofAl and having different thicknesses are formed on 36° Y-cutX-propagation LiTaO₃ substrates, and SiO₂ films having a film-thicknessof about 20% of the wavelength are formed by sputtering and CVD;

[0035]FIGS. 13A, 13B, and 13C are electron microscopic photographs ofstructures in which IDTs made of Al and having film-thicknesses of about0.02λ, about 0.04λ, and about 0.08λ are formed on 36° Y-cutX-propagation LiTaO₃ substrates, and an SiO₂ film having a thickness ofabout 30% of the wavelength is formed thereon by sputtering; and

[0036]FIG. 14 is an electron microscopic photograph showing across-section of a surface acoustic wave resonator in which IDTs made ofAl and having a film-thickness of about 0.1λ is formed on a 36° Y-cutX-propagation LiTaO₃ substrate, and an SiO₂ film having a thickness ofabout 30% of the wavelength is formed thereon by CVD.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Hereinafter, preferred embodiments of the present invention willbe described with reference to the accompanying drawings to make thepresent invention more clear.

[0038]FIG. 1A and FIG. 1B are a plan view showing a surface acousticwave device according to a preferred embodiment of the present inventionand a cross-sectional view thereof taken along line A-A in FIG. 1A,respectively.

[0039] The surface acoustic wave device 1 of the present preferredembodiment is preferably a one-port-type surface acoustic wave resonatorhaving reflectors. IDT 3 and reflectors 4 and 4′ are disposed on asubstantially rectangular plate-shaped piezoelectric substrate 2.Insulating films 5 made of SiO₂ are arranged on the piezoelectricsubstrate so as to cover the electrode films constituting the IDT 3 andthe reflectors 4 and 4′. In this preferred embodiment, thefilm-thicknesses of the electrode films are preferably in the range ofabout 1% to about 3% of the wavelength of an excited wave. Thereby, thedeterioration of the resonance characteristic and the filtercharacteristic can be effectively suppressed, even if the surface 5 a ofthe SiO₂ film 5 has convexities and concavities. This will be describedmore specifically with reference to examples below.

[0040] In known RF surface acoustic wave devices, electrode films madeof Al or an Al-base alloy are formed on a piezoelectric substrate, e.g.,an LiTaO₃ substrate or an LiNbO₃ substrate. However, the rotation-Y-cutLiTaO₃ substrates or LiNbO₃ substrates have problems in that theirfrequency-temperature coefficient TCF are high, i.e., are in the rangeof about −40 ppm/° C. to about −100 ppm/° C.

[0041] According to a known method which is effective in reducing thefrequency-temperature coefficient TCF, an SiO₂ film is arranged so as tocover electrode films disposed on a piezoelectric substrate (theabove-described Patent Documents 1 to 4).

[0042] However, surface acoustic wave devices having SiO₂ films disposedthereon as described above have not been made fit for practical uses assurface acoustic wave devices to be operated in RF bands. Generally, insurface acoustic wave filters for operation in RF bands and surfaceacoustic wave DPXs, ladder-type filters are used in which a plurality ofone-port type surface acoustic wave resonators are connected in so as todefine a ladder-type circuit. In these ladder-type filters, electrodesare made of Al or an Al base alloy, and the film-thicknesses of theelectrodes are thick, i.e., are set at values equal to about 8% to about10% of the wavelength of a surface acoustic wave. This is carried out toobtain a sufficient reflection characteristic and a satisfactoryelectromechanical coupling coefficient.

[0043] Practically, one-port type surface acoustic wave resonatorshaving electrodes made of Al with a thickness equal to about 8% of thewavelength of a surface acoustic wave were produced, in which SiO₂ filmswith different film-thicknesses were formed so as to cover the electrodefilms. Thus, the change of the characteristics was determined. FIG. 2shows the results. The characteristics shown in FIG. 2 are those of thesurface acoustic wave resonator of which the structure is schematicallyshown in the cross-section in FIG. 3. In particular, the surfaceacoustic wave resonator 11 has a structure in which IDTs 13 made of Aland one pair of reflectors (not shown) are formed on a piezoelectricsubstrate 12 made of a 36° Y-cut X-propagation LtTaO₃. Moreover, SiO₂films 15 are arranged so as to cover the IDTs 13 and the reflectors.

[0044] As seen in FIG. 2, the formation of the SiO₂ films 15 causes theresonance characteristics to be considerably deteriorated. Moreover, itis seen that the larger the thickness of the SiO₂ film 15 is, the morethe deterioration degree becomes.

[0045] Moreover, as seen in FIG. 3, large convex portions and concaveportions are formed on the surface of the SiO₂ film 15. The convexportions and concave portions are formed, since the portions of the SiO₂film existing above the IDTs and the reflectors are raised compared tothe other portions of the SiO₂ film.

[0046] In practice, the shapes of the convex portions and concaveportions are considerably different, depending on the portions of theSiO₂ film, as seen the scanning electron microscopic photographs of FIG.4A and FIG. 4B. Probably, this is because film-forming particles arejetted toward the piezoelectric substrate not only in the verticaldirection but also in an oblique direction when the SiO₂ film is formed.That is, the SiO₂ film formed of the particles jetted in an obliquedirection is grown in the oblique direction, so that the surface of theSiO₂ film becomes irregular. Probably, it is very difficult to controlthe irregularities of the convex and concave shapes as described above.

[0047] As described above, the deterioration of the characteristic ofthe known surface acoustic wave resonator 11 having the SiO₂ film 15formed therein is caused by the convex portions and concave portionsexisting on the surface of the SiO₂ film 15 and also the irregularitiesin the convex and concave shapes. The convex and concave shapes dependon the shapes of the electrodes and the film-thickness.

[0048] Thus, the change of the characteristics of a one-port typesurface acoustic wave resonator and a ladder type filter, caused by theformation of SiO₂ films, was determined. That is, the characteristicswere determined before and after the formation of the SiO₂ films, withthe film-thickness of an electrode in the range from about 2% to about8% of the wavelength of a surface acoustic wave and the film-thicknessof the SiO₂ film being in the range from about 10% to about 30% of thewavelength. The one-port type surface acoustic wave resonator used forthe evaluation was formed in a manner similar to the one-port typesurface acoustic wave resonator 11 shown in FIG. 3. The SiO₂ film wasformed by sputtering. The sputtering conditions are preferably asfollows: the obtained vacuum was about 1×10⁻⁵ Pa to about 5×10⁻⁴ Pa; thepower was about 0.5 kW to about 1.5 kW; the gas pressure was about 0.2Pa to about 0.4 Pa; and the heating temperature was about 100° C. toabout 300° C.

[0049] The change of the characteristic of the one-port type surfaceacoustic wave resonator in which the film-thickness of an electrode filmmade of Al is equal to about 2% of the wavelength of a surface acousticwave, caused by the different film-thicknesses of SiO₂ films, wasdetermined. FIG. 5 shows the results. FIGS. 6A and 6B are scanningelectron microscopic photographs showing the shape in cross-sections ofthe surface acoustic wave resonator.

[0050] As seen by comparison of FIGS. 6A and 6B to FIGS. 4A and 4B, theconvex portions and concave portions on the surface of the SiO₂ film aredecreased correspondingly to the reduction in thickness of the Al film.However, in this case, the formation of the SiO₂ film also causes theresonance characteristic to change significantly as seen in FIG. 5.

[0051] As seen by comparison of FIG. 5 to FIG. 2, when thefilm-thickness of the electrode film made of Al is about 2%, the ratiosof the tops to the bottoms, i.e., the ratios of the anti-resonanceresistances to the resonance resistances are not deteriorated, althoughthe resonant frequencies are varied. That is, it can be seen that theresonance characteristics are superior. Probably, this is because thethickness of the electrode film is small, so that the convex portionsand concave portions on the surface of the SiO₂ film become small.

[0052] In addition, when the thickness of the Al electrode film issmall, i.e., about 2%, no formation of the SiO₂ films causes largeripples to be generated in the vicinities to the anti-resonance points.On the other hand, the formation of the SiO₂ films prevents such ripplesfrom being generated. Probably, this is due to the increase of thereflection quantity caused by the convex portions and concave portionson the film of the SiO₂ films.

[0053] That is, as seen in FIG. 5, when the thickness of the Alelectrode film is small, the convex portions and concave portions on thesurface of the SiO₂ films are reduced in size. Thereby, thedeterioration of the resonance characteristic is prevented, andmoreover, the SiO₂ films are effective in preventing ripples from beinggenerated in the vicinities to the anti-resonance points.

[0054] The above-described convex portions and concave portions on thesurfaces of the SiO₂ films mean those of which the difference betweenthe heights of a concavity and a convexity is in the range of about 70%to about 130% of the thickness of the electrode film. Probably, thevariation of the height of the surface of the insulating film in thisrange is due to the growth of particles jetted in an oblique directionwhich was made when the SiO₂ films were formed by sputtering asdescribed above.

[0055] A ladder type filter including a plurality of one-port typesurface acoustic wave resonators connected so as to obtain a ladder typecircuit configuration was produced based on the above-describedexperimental results of the one-port type surface acoustic waveresonator. The thicknesses of electrode films and the variations ofinsertion loss were determined. In this case, a ladder type filter wasproduced, in which three series-arm resonators and two parallel-armresonators are connected so as to provide a ladder type circuit.

[0056] The IDTs and the reflectors of the ladder type filter were formedof Al.

[0057] In one-port type surface acoustic wave resonators constitutingthe above-described series-arm resonators and the parallel-armresonators, SiO₂ films were formed so as to have thicknesses equal toabout 10%, about 25%, and about 45% of the wavelength of a surfaceacoustic wave as in the above-described experiment. Plural types ofladder type filers were prepared by use of the one-port type surfaceacoustic wave resonators. FIG. 7 shows the experimental results.

[0058] As seen in FIG. 7, when the thickness of the electrode filmexceeds about 4%, the insertion losses of the filters are rapidlydeteriorated. Thus, desirably, the thickness of the electrode film isset at a value of up to about 4%. In particular, when an electrode isformed on a piezoelectric substrate, and an SiO₂ film is formed so as tocover the electrode, convex portions and concave portions are formed onthe surface of the SiO₂ film as described above. However, it can be seenthat the deterioration of the insertion loss of the ladder type filtercan be reliably prevented by setting the thickness of the electrode filmat a value of up to about 4%, preferably, at a value of up to about 3%.

[0059] In the above-described experiment, the electrode film was formedof Al. As confirmed by the inventors of the present invention, when theelectrode film was formed of a heavier metal than Al such as Au, Cu, Ag,W, Ta, Pt, Mo, Ni, Co, Cr, Fe, Mn, Zn, Ti or other suitable material,results similar to those as described above were obtained.

[0060] It is known that the characteristic of such a device having aSiO₂ film formed thereon can be improved by flattening the upper surfaceof the SiO₂ film (see Patent Document 4), as described above. However, afilm-forming method in which the upper surface of a SiO₂ film isflattened is complicated, or a further process of flattening isrequired. On the other hand, according to preferred embodiments of thepresent invention, the SiO₂ film can be easily formed by RF sputteringas described above. Thus, a complicated process is not required.

[0061] According to preferred embodiments of the present invention, amethod of flattening the surface of an insulating film such as the SiO₂film or the like may be also applied, and thereby, a resonancecharacteristic and a filter characteristic that is even more improvedcan be obtained.

[0062] If the thickness of the electrode film is excessively small, thereflection per one electrode finger may become insufficient, and theresistance of an electrode finger is rapidly increased. Thus, thethickness of the electrode film is preferably set at a value of up toabout 1% of the wavelength of a surface acoustic wave.

[0063] The above-described results have been obtained depending on thethickness of an electrode, the thickness of an SiO₂ film formed so as tocover the electrode, and the profile of the SiO₂ film on the surfacethereof. Accordingly, it is not only when a 36° Y-cut X-propagationLiTaO₃ substrate is used that the above-described effects can beobtained. Effects similar to those described above can be also obtainedwhen a piezoelectric substrate having another cut-angle and being madeof another material is used. This will be described more specificallywith reference to FIG. 8.

[0064] SiO₂ films having different film-thicknesses were formed onLiTaO₃ substrates having different cut-angles. Then, the relationshipbetween the thickness of the SiO₂ film and the frequency-temperaturecoefficient TCF was determined. FIG. 8 shows the experimental results.Moreover, SiO₂ having different film-thicknesses were formed on LiNbO₃substrates. The relationship between the film-thickness of the SiO₂ filmand the frequency-temperature coefficient TCF was determined. FIG. 9shows the experimental results.

[0065] As seen in FIGS. 8 and 9, the frequency-temperature coefficientTCF is linearly shifted toward the positive side with increasing of thethickness of the SiO₂ film in both of the case in which the LiTaO₃substrates are used and the case in which the LiNbO₃ substrates areused. Moreover, as seen in FIGS. 8 and 9, such a tendency as describedabove is found in the case in which the cut-angle is changed.

[0066] Thus, as seen in FIGS. 8 and 9, the frequency-temperaturecoefficient TCF can be decreased to one half of that obtained when noSiO₂ is formed, by setting the thickness of the SiO₂ film at a value inthe range of about 15% to about 40% of the wavelength of a surfaceacoustic wave. Moreover, preferably, the frequency-temperaturecoefficient TCF can be reduced substantially to zero by setting thethickness of the SiO₂ film at a value of at least about 30% of thewavelength of a surface acoustic wave. Furthermore, preferably, thefrequency-temperature coefficient TCF can be reduced substantially tozero by using LiNbO₃ substrates and LiTaO₃ substrates having cut-anglesof about 0 to about 160°.

[0067] It is to be noted that the frequency-temperature coefficient TCFscarcely changes when the thickness of the electrode film is varied.Accordingly, as seen in FIG. 7, a frequency-temperature coefficient TCFcan be even more improved while the insertion loss and the resonancecharacteristic are deteriorated, by setting the thickness of the SiO₂film at a value in the range of about 15% to about 40% of the wavelengthof a surface acoustic wave in the configuration of such a device inwhich the thickness of the electrode film is in the range of about 1% toabout 4% of the wavelength of the surface acoustic wave.

[0068] In preferred embodiments of the present invention, the type ofsurface acoustic wave used is not especially limited. Preferably, a Lovewave, which is known as a surface acoustic wave to be propagated on arotation Y-cut X-propagation LiNbO₃ substrate, is used. When a Love waveis propagated, substantially no attenuation is generated. Thus, aresonance characteristic and also a filter characteristic more improvedcan be obtained. To generate a Love wave as an excited surface acousticwave, the mass-addition is required in general. Thus, desirably, anelectrode is formed of a metal of which the specific gravity is higherthan Al.

[0069] Then, electrodes made of different metals were formed on rotationY-cut X-propagation LiNbO₃ substrates. Moreover, SiO₂ films were formedon the surfaces. The relationship between the thickness of the electrodefilm and the electromechanical coupling coefficient was determined.FIGS. 10 and 11 show the measurement results.

[0070] In the measurement of which the results are shown in FIG. 10, anLiNbO₃ substrate with an Euler's angle (0, 154, 0) was used. A leakysurface acoustic wave (LSAW) was used as a thin surface acoustic wave.In the measurement of which the results are shown in FIG. 11, a Rayleighwave which is propagated on a LiNbO₃ substrate with an Euler's angle (0,131, 0) and moreover, a leak surface acoustic wave were used.

[0071] As seen in FIGS. 10 and 11, when the electrode film is formed ofa metal heavier than Al, the electro-mechanical coupling coefficient hasa maximum in the region of the film-thickness which is smaller thanabout 6% of the wavelength of the surface acoustic wave, morespecifically, in the region of the film-thickness which is smaller thanabout 4% of the wavelength, and thus, has a large electromechanicalcoupling coefficient compared to a known electrode made of Al. Moreover,it is seen that for an electrode made of a metal heavier than Al, thechange of the electromechanical coupling coefficient with thefilm-thickness is larger than that for the Al electrode.

[0072] Generally, desirably, the electromechanical coupling coefficienthas a value suitable for a desired filter-bandwidth. As described above,when a metal heavier than Al is used, the adjustable range of theelectromechanical coupling coefficient is wide, so that anelectromechanical coupling coefficient corresponding to a desiredbandwidth can be easily obtained.

[0073] In the description above, an SiO₂ film is preferably used as theinsulating film. Piezoelectric substrates made of Ta₂O₃ and ZnO may beused to improve the temperature characteristic, enhance thepiezoelectric property, and protect the surface of a device. In thiscase, according to preferred embodiments of the present invention,deterioration of the resonance characteristics, thefilter-characteristics, and so forth can be prevented as in theabove-described examples, while the piezoelectric property can beimproved, and the surface-protection effect can be enhanced. Also, thedeterioration of the characteristics of such a device, which is causedby the formation of the insulating film made of a piezoelectric film orthe like, can be prevented by setting the thickness of the electrodefilm at a value in the range of about 1% to about 3%.

[0074] In the case in which electrodes of Al are formed on apiezoelectric substrate to form a one-port type surface acoustic waveresonator, and an SiO₂ film is arranged so as to cover the electrodefilms, as shown in FIG. 4B, large convex portions and concave portionstend to be formed on the surface of the SiO₂ film, caused by theformation of the electrodes for the IDTs and the reflectors. Such convexportions and concave portions can be reduced in size by selection of anappropriate method of forming an SiO₂ film. This will be described withreference to FIGS. 12 to 14.

[0075] Interdigital electrodes of Al having different film-thicknesseswere formed on 36° Y-cut X-propagation LiTaO₃ substrates, and SiO₂ filmshaving a film-thickness of about 0.2λ were formed thereon by sputteringand by CVD. The change of the reflection coefficients caused by theformation of the SiO₂ films was measured. FIG. 12 shows the measurementresults.

[0076] As seen in FIG. 12, when the thicknesses of the IDTs made of Alare increased, that is, the heights of the convex portions formed on thesurfaces of the SiO₂ films are increased, the reflection coefficientsbecome large. For the SiO₂ film formed by CVD, the reflectioncoefficient is smaller that that for the SiO₂ film formed by sputtering.A probable reason for this is as follows. When the SiO₂ is formed bysputtering, the surface of the SiO₂ film has convex portions and concaveportions which substantially conform to the convex portions and concaveportions formed by the presence of the electrodes. On the other hand,when the SiO₂ film is formed by CVD, the surface of the SiO₂ film hascurved portions which are formed due to the upward projection of theelectrodes present under the curved portions. Thus, the surface of theSiO₂ film becomes curved between the convex portions at the surface ofthe SiO₂ film. Hence, the difference in height between the concaveportions and convex portions becomes small.

[0077] IDTs of Al having thicknesses of about 0.02λ, about 0.04λ, andabout 0.08λ were formed on LiTaO₃ substrates, and SiO₂ films having athickness of about 0.3λ were formed. Thus, surface acoustic waveresonators were formed. FIGS. 13A, 13B, and 13C show the electronmicroscopic photographs showing the cross-sectional structures of thesurface acoustic wave resonators. Moreover, IDT of Al having a thicknessof about 0.1λ was formed, and an SiO₂ film having a thickness of about0.3λ (λ represent the wavelength) was formed. Thus, a surface acousticwave resonator was formed. FIG. 14 shows an electron microscopicphotograph showing the cross-sectional structure of the surface acousticwave resonator.

[0078] As seen by the comparison of FIGS. 13A, 13B, and 13C with FIG.14, in the case of the SiO₂ films formed by CVD, the difference inheight between the convex portions and the concave portions on thesurface of the SiO₂ films is small compared to the SiO₂ films formed bysputtering.

[0079] The present invention is not limited to each of theabove-described preferred embodiments, and various modifications arepossible within the range described in the claims. An embodimentobtained by appropriately combining technical features disclosed in eachof the different preferred embodiments is included in the technicalscope of the present invention.

What is claimed is:
 1. A surface acoustic wave device comprising: apiezoelectric substrate; electrode films disposed on the piezoelectricsubstrate and constituting at least one IDT; and a sputtered insulatingfilm arranged on the piezoelectric substrate so as to cover theelectrode films and the insulating film having convex portions andconcave portions on an upper surface thereof; wherein a film-thicknessof the electrode films is in the range of about 1% to about 3% of thewavelength of an excited surface acoustic wave.
 2. A surface acousticwave device according to claim 1, wherein the insulating film is made ofSiO₂.
 3. A surface acoustic wave device according to claim 2, whereinthe film-thickness of the insulating film made of SiO₂ is in the rangeof about 15% to about 40% of the wavelength of the surface acousticwave.
 4. A surface acoustic wave device according to claim 3, whereinthe film-thickness of the insulating film made of SiO₂ is in the rangeof at least about 30% of the wavelength of the surface acoustic wave. 5.A surface acoustic wave device according to claim 1, wherein thepiezoelectric substrate has a frequency-temperature coefficient TCFwhich is in the range of about −100 ppm/° C. to about −10 ppm/° C.
 6. Asurface acoustic wave device according to claim 1, wherein thepiezoelectric substrate is a rotation Y-cut X-propagation LiTaO₃substrate or a rotation Y-cut X-propagation LiNbO₃ substrate.
 7. Asurface acoustic wave device according to claim 6, wherein the LiTaO₃substrate has a cut-angle which is in the range of about 0° to about160°.
 8. A surface acoustic wave device according to claim 1, whereinthe piezoelectric substrate is a rotation Y-cut X-propagation LiNbO₃substrate, and the surface acoustic wave is a Love wave.
 9. A surfaceacoustic wave device according to claim 8, wherein the LiNbO₃ substratehas a cut-angle which is in the range of about 0° to about 160°.
 10. Asurface acoustic wave device according to claim 1, further comprising areflector defined by the electrode films.
 11. A surface acoustic wavedevice according to claim 1, wherein the electrode films are made of Alor an Al alloy.
 12. A surface acoustic wave device according to claim 1,wherein the electrode films are made of a metal or an alloy having ahigher density than Al.
 13. A surface acoustic wave device according toclaim 1, wherein the surface acoustic wave device is a one-port typesurface acoustic wave resonator.
 14. A surface acoustic wave deviceaccording to claim 1, wherein the surface acoustic wave device is aresonator type filter, a ladder type filter, or a lattice type filter.15. A method of forming a surface acoustic wave device, the methodcomprising the steps of: providing a piezoelectric substrate; formingelectrode films on the piezoelectric substrate so as to form at leastone IDT; and sputtering an insulating film on the piezoelectricsubstrate so that the sputtered insulating film covers the electrodefilms and so that the insulating film has convex portions and concaveportions on an upper surface thereof; wherein a film-thickness of theelectrode films is in the range of about 1% to about 3% of thewavelength of an excited surface acoustic wave.
 16. A method accordingto claim 15, wherein the sputtering is carried out at a vacuum of about1×10⁻⁵ Pa to about 5×10⁻⁴ Pa, a power of about 0.5 kW to about 1.5 kW, agas pressure of about 0.2 Pa to about 0.4 Pa, and a heating temperatureof about 100° C. to about 300° C.
 17. A method according to claim 15,wherein the insulating film is made of SiO₂.
 18. A method according toclaim 17, wherein the film-thickness of the insulating film made of SiO₂is in the range of about 15% to about 40% of the wavelength of thesurface acoustic wave.
 19. A method according to claim 18, wherein thefilm-thickness of the insulating film made of SiO₂ is in the range of atleast about 30% of the wavelength of the surface acoustic wave.
 20. Amethod according to claim 15, wherein the piezoelectric substrate has afrequency-temperature coefficient TCF which is in the range of about−100 ppm/° C. to about −10 ppm/° C.